1. Field of the Invention
The present invention relates to a method and an apparatus for controlling the gray scale of a plasma display devices and more particularly, to a method and an apparatus for controlling the gray scale of a three-electrode surface-discharge alternating current plasma display device.
2. Description of the Related Art
In the prior art, there has been known an alternating current plasma display panel (AC PDP) in which luminescence and display is done by applying a voltage wave-form to two sustain electrodes alternately to maintain discharge. In this AC PDP, a discharge operation is carried out in one to several microseconds (.mu.s) just after the pulse application. Further, ions (positive charges) produced by the discharge accumulate on the surface of the dielectric layer on the electrode to which a negative voltage is being applied and similarly electrons (negative charges) accumulate on the surface of the dielectric layer of the electrode to which a positive voltage is being applied.
When applying a pulse (sustain pulse) of a lower voltage (sustain voltage or sustain discharge voltage) with a different polarity after first discharging with higher voltage (write voltage) pulse (write pulse) to produce wall charges, previously accumulated wall charges are overlapped yielding a high voltage with respect to the discharge space, the voltage exceeding the threshold voltage value of discharges which causes a discharge to begin. That is, there is a characteristic that once a cell is written to discharge generated wall charges, the discharge is sustained by applying sustain pulses alternately in opposite polarity. It is called a memory effect or a memory function
Generally, an AC PDP makes use of the memory effect. Recently, as to AC PDPs, there has been proposed a two-electrode type in which selective discharge (address discharge) and sustain discharge are carried out with two electrodes, and a three-electrode type in which the third electrode is used for address discharge. In a color PDP used for a color displays a phosphor formed in a discharge cell is excited by ultraviolet rays generated by the discharge. However, there is a disadvantage that the phosphor is easily affected by bombardment of ions (positive charges) generated concurrently by discharge.
In the above mentioned two-electrode type, the arrangement is such that ions strike directly against phosphors, which is likely to lead to a reduction in the life of the phosphors. In order to avoid this, a three-electrode arrangement is generally used making use of surface discharge in a color PDP. Further, in such a three-electrode type, there are cases of forming a third electrode on the substrate on which the first and second electrode for sustain discharge is disposed and of forming it on another substrate facing the former. Also, in case of forming the said third electrode on the same substrate, there are the cases of disposing the third electrode on the two electrodes for sustain discharge and of disposing it under them. Furthermore, in some cases visible light emitted from phosphors is viewed through the phosphors, and in the other cases reflected light from the phosphors is viewed. In this specification, explanations are given taking an example of a panel in which the third electrode is formed on the substrate different from and facing that of electrodes for sustain discharge.
By the ways recently, higher level gray scales in many display lines have become necessary in an AC PDP with the requirements of a larger display size, a larger number of pixels (cells) and full color display in a display panel. Furthers it is required for an AC PDP to control the gray scale thereof by desired brightness, or appropriate brightness.